[Space Lua] tostring() respects __tostring metamethod with live stack frame

`tostring()` was delegating to `luaToString()` which calls `__tostring`
with a dead stack frame, breaking metamethods that call back into
globals. Now `tostringFunction` receives the live stack frame, looks up
`__tostring` via `rawGet`, calls it through `luaCall` with the live
frame, and validates the return is a string.

Also add tests covering the basic case, the nested-call case, and the
error path for a non-string return.

Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>

client/space_lua/stdlib/global_test.lua

@@ -25,3 +25,30 @@ assertEqual(some(" \n"), nil)
 assertEqual(some({}), nil)
 assertEqual(some({"baz"})[1], "baz") -- compare an element to ensure passthrough
 assertEqual(some({foo="bar"})["foo"], "bar")
+
+-- tostring: primitives
+assertEqual(tostring(nil), "nil")
+assertEqual(tostring(true), "true")
+assertEqual(tostring(false), "false")
+assertEqual(tostring(42), "42")
+assertEqual(tostring(3.14), "3.14")
+
+-- tostring: `__tostring` metamethod is called with the live SF
+local mt = { __tostring = function(t) return "custom:" .. t.name end }
+local obj = setmetatable({ name = "lua" }, mt)
+assertEqual(tostring(obj), "custom:lua")
+
+-- tostring: `__tostring` returning non-string must error
+local bad_mt = { __tostring = function(_) return 99 end }
+local bad_obj = setmetatable({}, bad_mt)
+local ok, err = pcall(tostring, bad_obj)
+assertEqual(ok, false)
+
+-- tostring: `__tostring` on a nested call (SF propagation via `luaCall`)
+local inner_mt = {
+    __tostring = function(t)
+        return "inner:" .. tostring(t.val)
+    end
+}
+local inner = setmetatable({ val = 7 }, inner_mt)
+assertEqual(tostring(inner), "inner:7")